Cell cycle-related fluctuations in transcellular ionic currents and plasma membrane Ca2+/Mg2+ ATPase activity during early cleavages of Lymnaea stagnalis embryos.

During the first four mitotic division cycles of Lymnaea stagnalis embryos, we have detected cell cycle-dependent changes in the pattern of transcellular ionic currents and membrane-bound Ca2+-stimulated ATPase activity. Ionic currents ranging from 0.05 to 2.50 μA/cm2 have been measured using the vibrating probe technique. Enzyme activity was detected using Ando's cytochemical method (Ando et al. 1981) which reveals Ca2+/Mg2+ ATPase localization at the ultrastructural level, and under high-stringency conditions with respect to calcium availability, it reveals Ca2+-stimulated ATPase. The ionic currents and Ca2+-stimulated ATPase localization have in common that important changes occur during the M-phase of the cell cycles. Minimal outward current at the vegetal pole coincides with metaphase/anaphase. Maximal inward current at the animal pole coincides with the onset of cytokinesis at that pole. Ca2+-stimulated ATPase is absent from one half of the embryo at metaphase/anaphase of the two- and four-cell stage, whereas it is present in all cells during the remaining part of the cell cycle. Since fluctuations of cytosolic free calcium concentrations appear to correlate with both karyokinesis and cytokinesis, we speculate that part of the cyclic pattern of Ca2+-stimulated ATPase localization and of the transcellular ionic currents reflects the elevation of cytosolic free calcium concentration during the M-phase.